A carbon fiber is a material to which many applications have been made as a composite material with a resin, in order to increase strength of the resin and to buffer brittle fracture of the carbon fiber with the resin. However, the carbon fiber is commonly used as a fiber bundle constituted by many filaments and has low elongation, so that there have been problems that fluffs due to mechanical friction or the like are occurred and the filament fibers are prone to be breaked. Accordingly, when the carbon fiber is used, it is general to impart a sizing agent to a surface of the carbon fiber, in order to improve bundling properties and handleability and to improve adhesiveness between the fiber and the resin. This is because the use of such a sizing agent decreases the fluffs and the breakage of the fiber, which makes it possible to optimize a reinforcing effect of the carbon fiber.
For example, Patent Document 1 discloses a technique of improving interfacial adhesive strength using an epoxy resin-based sizing agent. As long as a matrix resin to be reinforced with the carbon fiber bundle is a thermosetting resin, it is not so bad. However, in the case of using a thermoplastic resin, there has been a problem of failing to obtain high adhesive strength generally because of poor compatibility between the resin and the sizing agent.
Accordingly, in order to enhance the compatibility, there is also considered a method of performing sizing using a thermoplastic resin-based sizing agent. However, the thermoplastic resin-based sizing agent is generally liable to harden a texture, although it improves interfacial adhesive strength, and significantly deteriorates handleability and processability. Physical properties of a composite finally obtained have been insufficient. For example, Patent Document 2 discloses a method of imparting an acid-modified polyolefin-based sizing agent in the case of using polypropylene that is a thermoplastic resin as the matrix resin. However, although an improvement in interfacial adhesive strength has been observed, processability in cutting or opening has been insufficient.
Further, in the case of using an engineering plastic resin (for example, a polyamide, a polyester, a polycarbonate or the like) having excellent physical properties as the thermoplastic resin, molding is generally performed between the melting point of the thermoplastic resin and the decomposition temperature thereof. However, there has been a problem that impregnation of the matrix resin into the fiber bundle is difficult because of high viscosity of the matrix resin within such a temperature range. Low viscosity as in the case of the conventional thermosetting resin is not obtained, so that fluidity of the thermoplastic resin becomes insufficient. As a result, it has had a problem that short-time molding of the composite becomes difficult.
In order to solve these problems, there is also considered a method of imparting a plasticizer and a sizing agent to the fiber at the same time. However, there has been a problem that such a plasticizer generally has an adverse effect on the matrix resin or adhesion thereof with the fiber, resulting in decreases in composite physical properties.
[Patent Document 1] JP-A-7-197381
[Patent Document 2] JP-A-2006-124847